Imagine for a moment that planet Earth isn’t running out of anything. We have plenty of food, plenty of oil, plenty of rare minerals, and plenty of air. In this little utopia, the only constraint is space. We can breed like bunnies, and everything is fine – until we hit what I call Peak Elbowroom.

This is more or less the idea behind a series of experiments conducted by John B. Calhoun in the 1960s. Calhoun offered a group of rats a limitless supply of food, water, bedding, and everything else healthy, happy rats could want – except space. He kept his rats confined in “rat cities” – elaborately partitioned boxes designed to simulate the urban environment, which he built in his basement in Washington, D.C.

So what was the rat response? Turns out they all died. Well, they went big, then died. The population spiked and plummeted in a blaze of rodent self-extermination.

It wasn’t for lack of resources, obviously, but just from the sheer stress of it all. They were fat, happy, and multiplying. Then Peak Elbowroom hit, and then they went berserk, became anti-social, stopped breeding, and then … Poof. Gone. Completely extinct. Calhoun repeated the experiment several times, always with the same results.